The first ionisation potential of `Na,Mg,Al` and `Si` are in the order

Options

A. `Na lt Mg gt AI lt Si`
B. `Na gt Mg gt AI gt Si`
C. `Na lt Mg lt AI lt Si`
D. `Na gt Mg gt AI gt Si`

Answer

A

Explanation

Across the period (i.e. `3^(rd)` period) the size of atom decreases and nuclear charge increases. So generally the ionisation energy increases. However the ionisation energy of `Mg` is greater than `AI` because of more penetration power of `2s` sub-shell electrons of `Mg` as compared to that of the `2p` sub-shell electron of `AI`.